The buildup of a polymer filter cake on the fracture face during hydraulic fracturing results in a decrease in the rate that water leaks off into the formation. The ability to predict this fluid loss is crucial to correctly design pumping schedules and to predict fracture geometry. It is also critical for the interpretation of the pressure and flow rate logs during fracture treatment. Laboratory tests of dynamic fluid loss are time consuming, and it is not possible to simulate in the laboratory the filter cake characteristics that occur far from the well bore. We have begun a fundamental study of filter cake formation that considers the micro-scale processes that govern cake formation. Filter cake formation is determined by the polymer concentration dependence of the permeability, osmotic pressure, and yield stress in the filter cake --all of which can be independently measured. This model will predict the polymer concentration profile in the filter cake, filter cake thickness, and fluid leakoff as a function of pressure drop across the filter cake. To verify this model we have developed a single fiber fluorescence probe to directly measure the polymer concentration in situ during filter cake deposition. Measurements of the concentration profile are presented for dilute HPG (hydroxypropyl guar) solutions and for 0.48 wt% (40 lb/Mgal) solutions.

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